Recent discoveries from red optical proper motion and wide-field near-infrared surveys have uncovered a new population of ultracool subdwarfs, metal-poor stars and brown dwarfs extending into the late-type M, L and possibly T spectral classes. These objects are among the first low-mass stars and brown dwarfs formed in the Galaxy, and are valuable tracers of metallicity effects in low-temperature atmospheres. Like solar metallicity late-type dwarfs, ultracool subdwarfs emit the majority of their emergent flux at infrared wavelengths. Here I discuss how Spitzer observations will contribute to the study of these objects, enabling measurement of the temperature scale, tests for dust formation and the identification of surface gravity and metallicity diagnostics in the coldest brown dwarfs.